Round and Turn are not synonyms.
From Player's Basic Rules, page 69, or the Player's Handbook, page 189 (bolding added for emphasis):
A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn.
"On your turn" thus specifically excludes other characters' turns.
In the sections on Actions and Bonus Actions, it defines when each is allowed.
On your turn, you can move a distance up to your speed and take one action.
[…]
Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action.
Then, in the section on the ready action (PBR 72, PHB 193)...
To be readied, a spell must
have a casting time of 1 action, and holding onto the
spell’s magic requires concentration (explained in
chapter 10).
A casting time of "1 bonus action" is not the same as "1 Action". In fact, the general casting time is 1 action; a bonus action is specifically called out as different under "Casting Time" on PBR 79 and PHB 202...
Bonus Action
A spell cast with a bonus action is especially swift. You
must use a bonus action on your turn to cast the spell,
provided that you haven’t already taken a bonus action
this turn.
So, in looking at the letter of the rules, it's clear that you can't use a bonus action spell outside your turn, and further, it's explicit that your turn ends when someone else begins their turn.
This is how skills are supposed to work!
If you are in a situation where there is only one person doing something, and they are rolling a single skill check, then yes, this is how it's supposed to work. Giving help is a natural thing and should be used in situations like this. There is no reason to prevent it unless the task is clearly something that's not going to benefit from someone else giving you assistance. There are some things you can do to limit it.
It's also worth noting that helping can often save you some table time. As a AceCalhoun points out in the comments, in many cases what happens if you don't help is that everyone in the party tries their hand at the task. This behaves very much like advantage, but with a slightly lower overall modifier (because most likely you'll have one character who is good at a task and the rest that are lower). So Working together only raises the change of success slightly and consumes less table time in these cases.
Be a bit more stringent about what you allow for assistance. Is coaching stealth really all that helpful? do you really want someone looking/talking over your shoulder while you're picking that lock? Evaluate situations where characters attempt to aid more carefully.
Have more than one thing going on at once. If all the characters need to be stealthy, they can't be helping each other. And if you need two arcane characters working on the sigils on opposite sides of the room, maybe they have to choose which one gets help from the third (or don't have anyone to help at all).
Make things take multiple rolls and limit helping on all of them. Maybe the first roll the wizard can be helped, but after that he's on the other side of the trap, or arm deep in the sigil or something to where additional assistance isn't going to help him.
Figure out how to inflict disadvantage for the task. Maybe there are mitigating circumstances.
Create a distraction. A rogue can't help the wizard if he's busy fighting baddies. Make some skill checks happen in an occupied room. Make completing the skill checks the win condition rather than defeating the enemies.
The basic crux of all of this is that helping is supposed to a mundane task that provides advantage. Yes, that's a huge deal, but it also doesn't stack with other things that give you advantage and it can be easily cancelled by disadvantage.
So get creative! Build some situations into your adventures that prevent your heroes from helping each other (or make the opportunity cost higher). But don't do it all the time, that might get tiresome. Adventurers like to help each other out, let them, but don't make it easy all the time.
Best Answer
There is currently no way to do this without assistance from the character sheet owner or rebuilding the character yourself.
Obviously you could just build the character yourself, but that takes time.
Alternatively, if you know the owner of the character sheet, you could ask them to copy the character and leave it unassigned in a campaign for you to claim. Otherwise, you're stuck taking some time rebuilding the sheet from scratch.